Photocatalytic hydrogen production is a possible answer to the present energy crisis. However, efficient hydrogen production requires the development of materials that demonstrate tunable optical response, have UV-visible activity, and are amenable to synthesis using simple methods. Metal oxides, mainly titanium dioxide (TiO2), have been studied for photocatalytic applications. Fujishima et al., Nature 1972, 238, 37; Hoffmann et al., Chem. Rev. 1995, 95, 69. However, decades of work with TiO2, such as to improve its properties via doping and addition of other photoactive materials, has demonstrated only incremental improvements. Martyanov et al., Chem. Commun., 2004, 2476; Anpo et al., J. Catal., 2003, 216, 505.
Pyrochlores (A2B2O7) are a lesser studied family of compounds that offer the flexibility to tune photocatalytic properties. Bismuth titanate (BTO), Bi2Ti2O7, is a photoactive member of the pyrochlore family that can potentially meet the aforementioned objectives desired of a photocatalyst. However, the full potential of this photocatalyst for hydrogen generation has not yet been fully exploited, which is partially because attempts to synthesize and characterize stoichiometric BTO pyrochlore are far and few. The earlier adopted synthesis methods have not always resulted in a pure pyrochlore phase. Yao et al., Appl. Catal. B Environ. 2004, 52, 109; Yao et al., Appl. Catal. A. Gen. 2003, 243, 185; Yao et al., J. Mol. Catal. A. 2003, 198, 343. For example, Radosavljevic et al. reported a method that resulted in a Bi—Ti—O composition of the form Bi1.74Ti2O6.62. Radosavljevic et al., J. Solid State Chem. 1998, 136, 63. Hector and Wiggin reported the synthesis and structural study of stoichiometric BTO along with some impurities of Bi4Ti3O12 using a co-precipitation method. Hector et al., J. Solid State Chem. 2004, 177, 139.
Recently, attempts to prepare stable crystalline BTO with a nanotube structure using anodized alumina as a template have been reported. Zhou et al., J. Mater. Res. 2006, 21, 2941. A template method, however, requires template removal, which is an additional step. To synthesize these materials economically on a large scale requires the development of a template-free method.
Accordingly, a need exists for new template-free methods of synthesizing pyrochlore nanostructures, such as pyrochlore nanorods.